Biasing and shielding circuit for source side sensing memory

ABSTRACT

A shielding circuit for preventing a sense current of a target cell from the influence of a source current of first adjacent cell includes a pre-discharge device, first and second biasing units, first and second voltage pull-down units, and a connection units. The pre-discharge device is for setting the voltage of the sense node to a negative voltage. The first and second biasing units are for biasing the source voltage of the target and the first adjacent cell equal to a biasing voltage, respectively. The first and second voltage pull-down units are for pulling down the source voltage of the target and the first adjacent cell closing to a ground level, respectively. The connection unit is for receiving and outputting the sense current passing through the first biasing unit to the sense node.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a biasing and shielding circuit, and more particularly to a shielding circuit applied in a source-side sensing memory.

2. Description of the Related Art

Along with increasing development of technology, non-volatile memory, such as flash memory, has been wildly used in electronic product. For the reading operation of memory data of a flash memory device, a sensing circuit is used to sense a cell current of a target cell for determining the memory data value stored in the target cell. Referring to FIG. 1, a circuit diagram of a cell matrix of a traditional flash memory device is shown. When a cell Ca is accessed, the cell current I1 of the cell Ca should be sensed so as to determine the data value stored in the cell Ca. When sensing, a sense circuit 100 is coupled to the cell Ca. Due to that cell current I1 is hard to be sensed and shielded, a sense current I2 flowing to the sense circuit 100 is sensed instead and the sense current I2 is used to know the magnitude of the cell current I1 of the cell Ca for the operation of sensing. The cell current I1 and the sense current I2 satisfied an equation:

I1=I2+I3

I3 is a leak current bypassing through the cell Cb, which is adjacent to the cell Ca. The leak current I3 will affect the preciseness of the sense current I1 and the correctness of the data value sensed by the sense circuit 100. The less difference of cell current and sensing current is, the more precisely the sense circuit 100 performs. We define the difference of cell current and sensing current to be error current. Therefore, how to design a flash memory circuit having small error current and precise data value sensing result has become an important goal to achieve.

SUMMARY OF THE INVENTION

The invention is directed to a biasing and shielding circuit, which biases the voltage level of the sources of a target cell and an adjacent cell substantially to the same voltage level. The biasing and shielding circuit can effectively prevent the cell current of the target cell from the influence of the source current of the transistor of the adjacent cell, thus enhance the preciseness of the data value sensing result, and enhance the sensing speed of the sense unit.

According to the present invention, a biasing and shielding circuit used in a memory for preventing a sense current of a target cell from the influence of a source current of first adjacent cell and providing the sense current to a sense node of a sense unit is provided. The biasing and shielding circuit comprises a pre-discharge device, first and second biasing units, first and second voltage pull-down units, and first connection units. The pre-discharge device is for setting the voltage level of the sense node to a negative voltage level when a first control signal is high. The first and second biasing units are for biasing the voltage level of the sources of the target cell and the first adjacent cell substantially equal to first biasing voltage level in response to second biasing voltage level, respectively. The sense current and the source current pass the first and the second biasing units, respectively, and the first biasing voltage level substantially closes to a ground level. The first and second voltage pull-down units are for pulling down the voltage level of the sources of the target cell and the first adjacent cell substantially closing to the ground level when a second control signal is negative low, respectively. The first connection unit is for receiving and outputting the sense current passing through the first biasing unit to the sense node when the second control signal is high, wherein, the drain of the first adjacent cell is coupled to the source of the target cell.

The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a cell matrix of a flash memory device.

FIG. 2 is a block diagram of a flash memory 10 according to a preferable embodiment of the invention.

FIG. 3 is a first detail circuit diagram of the biasing and shielding circuit 20 in the FIG. 2.

FIG. 4 is a signal waveform diagram of the shielding circuit 20 illustrated in FIG. 3.

FIG. 5 is a second detail circuit diagram of the biasing and shielding circuit 20 in the FIG. 2.

FIG. 6 is a third detail circuit diagram of the biasing and shielding circuit 20 in the FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The biasing and shielding circuit of this embodiment biases the source of a target cell and an adjacent cell to a biasing voltage level around the ground level, so as to prevent the sense current of the target cell from the influence of the source current of the adjacent cell next to the target cell.

Referring to FIG. 2, a block diagram of a flash memory 10 according to a preferable embodiment of the invention is shown. The flash memory 10 is a source-side sensing memory including a drain side bias circuit 12, a row decoder 14, a memory cell array 16, a shielding circuit 20 and a sense unit 22. The memory cell array 16 includes numerous memory cells arranged in an M×N matrix, and each of the memory cells includes a transistor. For example, a transistor T(i,j) is included in a target cell, wherein i is an integer not larger than M, j is an integer not larger than N, and M and N are nature numbers.

The drain side bias circuit 12 is coupled to N bit lines BL1˜BLN arranged along columns in the memory cell array 16 by a Y multiplexer 24 for providing drain bias signals DB to the drain side of the target cell or adjacent bit lines near by the drain side of the target cell in response to the address signals. The row decoder 14 is coupled to M word lines WL1˜WLM arranged along rows in the memory cell array 16 for providing word signals W to the cells of M rows according to the address signals. The transistor T(i,j) of a target cell is turned on and a source current I(i,j) is generated in response to the corresponding drain bias signal DB and the corresponding word signal W. The sense unit 22 determines the memory data value of the target cell according to a sense current IS(i,j) outputted by the corresponding bit line coupled to the source of the transistor T(i,j) of the target cell. The sense current IS(i,j) is substantially close to the source current I(i,j). For example, the source current I(1,3) flowing through transistor T(1,3) in response to the drain bias signal DB and word signal W delivered by the bit line BL4 and the word line WL1 respectively, and the sense current IS(1,3) corresponding to source current I(1, 3) is outputted by the bit line BL3.

The shielding circuit 20 prevents the sense current IS(i,j) from the influence of the source current I(i,j-1) of the transistor T(i,j-1) of the adjacent cell, and provides the sense current IS(i,j) to the sense unit 20. Next, the operation of the shielding circuit 20 is described by taking the memory cell including transistor T(1,3) for example

Referring to FIG. 3, a first detail circuit diagram of the shielding circuit 20 in the FIG. 2 is shown, and referring to FIG. 4, a signal waveform diagram of the shielding circuit 20 in FIG. 3 is shown. The sense unit 22 is, for example, a voltage detector. The voltage detector determines whether the voltage level at a sense node SN is high or low in comparison with a reference voltage level, so as to determine the memory data value of the target cell.

The shielding circuit 20 includes a pre-discharge device 202, biasing units 204 and 206, voltage pull-down units 208 and 210 and a connection unit 212. The pre-discharge device 202 is coupled to the sense node SN for controlling the voltage level of the sense node SN. The pre-discharge device 202 includes, for example, a transistor M1. The transistor M1 is an N type metal oxide semiconductor (MOS) transistor having a gate receiving the control signal SC1, a drain coupled to the sense node SN, and a source receiving the negative voltage level VLN. The transistor M1 is turned on and for pre-discharging the voltage level of the sense node SN to the negative voltage level VLN when the control signal SC1 is enabled in the first time period T1 and is turned off when the control signal SC1 in disabled in a second time period T2.

The biasing units 204 and 206 are coupled to the sources of the transistors T(1,3) and T(1,2) and is for biasing the voltage level of the sources of the transistors T(1,3) and T(1,2) substantially close to a biasing voltage level, respectively. The biasing units 204 and 206 include, for example, transistors M2 and M3, respectively. The transistors M2 and M3 are P type MOS transistors having gates receive the biasing voltage level VLB2, sources coupled to the sources of the transistors T(1,3) and T(1,2) respectively, and drains coupled to the voltage pull-down units 208 and 210 respectively. The transistor M2 and M3 is turned on in response to the biasing voltage level VLB2, so as to make the voltage level of the sources of the transistor M2 and M3 substantially satisfied the equations:

Ve _(—) M2=VLB2+Vth _(—) M2

Ve _(—) M3=VLB2+Vth _(—) M3

Ve_M2 and Ve_M3 are the voltage level of the sources of the transistors M2 and M3 and Vth_M2 and Vth_M3 are the threshold voltage of the transistors M2 and M3. The threshold voltage Vth_M2 and Vth_M3 are substantially the same. As a result, the voltage level of the sources of the transistors T(1,3) and T(1,2), are substantially the same. The biasing voltage level VLB2 is, for example, a negative voltage level, and the voltage level of the sources of the transistors M2 and M3 are both close to the ground level VLG. The sense currents IS(1,3) and IS(1,2) are passed through the transistors M2 and M3, respectively.

The voltage pull-down units 208 and 210 are coupled to the drains of the transistors M2 and M3 and for pulling down the voltage level of the drains of the transistors M2 and M3, respectively. The voltage pull-down units 208 and 210 include, for example, transistors M4 and M5, respectively. The transistors M4 and M5 are P type MOS transistors having gates receive a control signal SC2, drains receive the ground level VLG, and sources coupled to the drains of the transistors M2 and M3, respectively.

The transistors M4 and M5 are respectively turned on and for pulling down the voltage level of the drains of the transistors M2 and M3 substantially close to the ground level VLG when the control signal SC2 is enabled and negative in the first time period TP1. Therefore, voltage level of the source of the transistors M2 and M3 are substantially close to the ground level VLG and the voltage level of the source of the transistors T(1,3) and T(1,2) are also substantially close to the ground level VLG in the first time period TP1. The control signal SC2 is, for example, an inverse signal of the control signal SC1.

The connection unit 212 is coupled to the drain of the transistor M2 and the sense node SN for passing the sense current IS(1,3) flowing from the transistor M2 to charge the capacitance C1. The connection unit 212 includes, for example, a transistor M6. The transistor M6 is an N type MOS transistor having a gate receiving the control signal SC2, a source coupled to the sense node SN, and a drain coupled to the drain of the transistor M2. The transistor M6 is turned on so as to connect the drain of the transistor M2 to the sense node SN in the time period TP2. Therefore, the sense current IS(1,3) is passed to charge the capacitance C1, the voltage of the sense node SN varies with the sense current IS(1,3) multiplies with time and the data value of the target memory cell can be determined by the sense unit 22.

The cross voltage over the drain and source of the transistor T(1,2) is small because the voltage level of the sources of the transistor M3 and M2 are substantially close to each other. For example, the voltage level of the sources of the transistors M3 and M2 are 93 millivolt (mV) and 214 mV, respectively. Conventional the voltage level of the sources of the transistors M3 and M2 are 0 mill volt (mV) and 214 mV. The source current I(1,2), i.e. the leak current of source current (1, 3) is small due to the small cross voltage between source and drain of the transistor T(1,2). As a result, the biasing and shielding circuit 20 can effectively decrease the amount of the leak current to make the source current I(1,3) and the sense current IS(1,3) close to each other and the correctness rate of data value sensed by the sense circuit 22 is raised.

The transistor M2 is biased in saturation region in the second time period T2 because the source-drain voltage of transistor M2, which is close to 2.6 V(0−(−2.6)), is larger than the voltage difference between the gate-source voltage and the threshold voltage of the transistor M2. By using the transistor M2 operating in saturation region, the equivalent capacitance looking into the sense node SN is much smaller than the equivalent capacitance looking into the sense node in the traditional source side sensing memory. Therefore, the changing speed of the voltage at the sense node SN is enhanced, and the operational speed of sensing is effectively increased.

The biasing and shielding circuit 20 further includes a negative charge pump 222, level shift and logic units 214 and 216. The negative charge pump 222 provides the negative voltage level VLN to the pre-discharge unit 202 for setting the voltage level of the sense node SN to the negative voltage level VLN, which is substantially equal to −2.6 V. The level shift and logic units 214 and 216 provide the control signal SC1 and SC2 according to the negative voltage level VLN, respectively. The high and low level of the control signal SC1 and SC2 are substantially equal to the ground level VLG and VLN. The biasing and shielding circuit 20 further includes a regulator 224, which generates the biasing voltage level VLB2 in response to a reference signal Ref and a source signal NV. The source signal NV is, for example, provided by the negative charge pump 222 and has the negative voltage level VLN.

Referring to FIG. 2, the flash memory 10 according to the embodiment further includes a Y-multiplexer 18 coupled to the bit lines BL1˜BLN for receiving the sense currents of the corresponding N columns of memory cells. The Y-multiplexer 18 connects the corresponding bit lines BL3 and BL2 to the sources of the transistors M2 and M3 in response to selection signal bus SS. The selection signal is relevant to the address signal SA. For example, when the drain side biasing unit 12 and the row decoder 14 provide the corresponding drain bias signal DB and the word signal W to drive the transistor T(1,3), the y-multiplexer 18 couples the bit line BL3 and BL2 to the sources of the transistors M2 and M3, respectively. Therefore, the biasing and shielding circuit 20 is capable of biasing the voltage level of the sources of the transistors T(1,3) and T(1,2) close to the ground level VLG and transfer the sense current IS(1,3) to voltage passed to the sense unit 22 through the Y-multiplexer 18.

Although the operation of the Y-multiplexer 18 is exemplified to respectively connect the bit lines BL3 and BL2 to the source of the transistors M2 and M3, the Y-multiplexer 18 can further connect other bit lines to the source of the transistors M2 and M3 in response to the different selection signal bus SS′. Therefore, the sense unit 22 can sense the sense current corresponding to the source current of the cell on other columns of cells and the biasing and shielding circuit 20 can prevent the sense current corresponding to the target cell from the influence of the source current of the transistor of the adjacent cell.

Although the operation of the shielding circuit 20 is exemplified to prevent the sense current IS(1,3) of the target cell from the influence of the source current I(1,2), the operation of preventing other sense current from the influence of the corresponding source current is similar to the operation described above.

In the shielding circuit 20 of the embodiment, the biasing and the voltage pull-down units 206 and 210 form a shielding device for preventing the sense current of the target cell from the influence of the source current of the adjacent cell. Although the shielding circuit 20 of the embodiment is exemplified to have only one shielding device, the shielding circuit 20 of the embodiment is not limited to include only one shielding unit. For example, referred to FIG. 5, a second detailed circuit diagram of the shielding circuit 20 in the FIG. 2 is shown. In the biasing and shielding circuit 20′ shown in FIG. 5, another shielding device including a biasing unit 206′ and a voltage pull-down unit 210′ are added to the circuit of biasing and shielding circuit 20 mentioned above. The biasing unit 206′ and the voltage pull-down unit 210′ biases and pulls down the voltage level of the source of the transistor T(1,1). Therefore, the biasing and shielding circuit 20′ can further bias the voltage level of the source of the transistor T(1,1) for reducing the source current I(1,1) of the transistor T(1,1). The biasing and shielding circuit 20′ can also effectively prevent the sense current IS(1,3) from the influence of the source currents I(1,2) and I(1,3) and enhance the preciseness of the sense current IS(1,3).

In the biasing and shielding circuit 20, the transistors M2, M4 and M6 and capacitance C1 forms a sense device for providing the sense voltage of the target cell to the sense unit 22. Although the biasing and shielding circuit 20 of the embodiment is exemplified to have only one sense device, the biasing and shielding circuit 20 of the embodiment is not limited to include only one sense device. For example, referred to FIG. 6, a third detail circuit diagram of the biasing and shielding circuit 20 in the FIG. 2 is shown. In the biasing and shielding circuit 20″ shown in FIG. 6, anther connection units 212′ is added to the circuit of shielding circuit 20′ mention above. The biasing unit 206, the voltage pull-down unit 210, and the connection unit 212′ form another sense device provides both the sense current IS(1,2) and IS(1,3) to charge the sense node SN. Therefore, the biasing and shielding circuit 20″ can further provide the sense current IS(1,2), which is a component of the source current I(1,3) bypassing through the transistor T(1,2), to charge the sense node SN. The sense node SN is charged by the sum of the sense current IS(1,3) and IS(1,2) and the sense current sensed by the sense unit 22′ is much closer to the source current I(1,3) than the sense current sensed by the sense unit 22 in FIG. 3 and FIG. 5. Consequently, the biasing and shielding circuit 20″ can effectively improve the preciseness of the sense current sensed by the sense unit 22′.

The biasing and shielding circuits 20, 20′ and 20″ respectively having one sense and one shielding units, one sense and two shielding units, and two sense and one shielding units, have been cited above. However, the number of sense and shielding units included in the shielding circuit of the embodiment is not limited thereto.

The biasing and shielding circuit of this embodiment biases the sources of the transistors in a target cell and an adjacent cell to a biasing voltage level around the ground level. Therefore, the sense current of the target cell is effectively prevented from the influence of the source current of an adjacent cell and the preciseness of the data value sensing result of a traditional source side sensing flash memory device is effectively improved.

The biasing and shield apparatus of the embodiment uses a transistor M2, which biased in the saturation region when the sense current IS(1,3) is provided to the sense node SN. Therefore, the equivalent capacitance looking into the sense node SN is much smaller than the equivalent capacitance looking into the sense node in the traditional source side sensing flash memory and the changing speed of the voltage at the sense node SN and the operational speed of sensing are effectively increased.

While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. 

1. A biasing and shielding circuit used in a source side sensing memory for preventing a sense current of a target cell from the influence of a source current of a first adjacent cell and for providing the sense current to a sense node of a sense unit, the biasing and shielding circuit comprising: a pre-discharge device for setting the voltage level of the sense node to a negative voltage level when a first control signal is high enough to open the pre-discharge device; a first and a second biasing units for biasing the voltage level of the sources of the target cell and the first adjacent cell substantially equal to a first biasing voltage level in response to a second biasing voltage level respectively, wherein the sense current and the source current pass through the first and the second biasing units respectively, the first biasing voltage level substantially closes to a ground level; a first and a second voltage pull-down units for pulling down the voltage level of the sources of the target cell and the first adjacent cell substantially closing to the ground level when a second control signal is low respectively; and a first connection unit for receiving and outputting the sense current passing through the first biasing unit to the sense node when the second control signal is high; wherein the drain of the first adjacent cell is coupled to the source of the target cell.
 2. The biasing and shielding circuit according to claim 1, further comprising: a negative charge pump for providing the negative voltage level to the pre-discharge unit; a first level shift and logic unit for providing the first control signal according to the negative voltage level, wherein the first control signal is high and low in a first time period and a second time period respectively; and a second level shift and logic unit for providing the second control signal according to the negative voltage level, wherein the second control signal is high and low in the second period and the first time period respectively.
 3. The biasing and shielding circuit according to claim 1, wherein the pre-discharge unit comprises: a first transistor having a gate receiving the first control signal, a drain coupled to the sense node, and a source receiving the negative voltage level.
 4. The biasing and shielding circuit according to claim 1, wherein the first and the second biasing unit respectively comprises: a second transistor having a gate receiving the second biasing signal, a source coupled to the source of the target cell, and a drain coupled to the first voltage pull-down unit; and a third transistor having a gate receiving the second biasing signal, a source coupled to the source of the first adjacent cell, and a drain coupled to the second voltage pull-down unit.
 5. The biasing and shielding circuit according to claim 1, wherein the first and the second pull-down unit respectively comprises: a fourth transistor having a gate receiving the second control signal, a source coupled to the first biasing unit, and a drain receiving the ground level; and a fifth transistor having a gate receiving the second control signal, a source coupled to the second biasing unit, and a drain receiving the ground level.
 6. The biasing and shielding circuit according to claim 1, wherein the first connection unit comprises: a sixth transistor having a gate receiving the second control signal, a source coupled to the sense node and a drain coupled to the first biasing unit.
 7. The biasing and shielding circuit according to claim 1, wherein the sense node is coupled to a designed capacitance.
 8. The biasing and shielding circuit according to claim 1, further comprising: a third biasing unit for biasing the voltage level of a source of a second adjacent cell substantially equal to a first biasing voltage level in response to a second biasing voltage level, wherein the source current of the second adjacent cell pass the third biasing unit; and a third voltage pull-down unit for pulling down the voltage level of the source of the second adjacent cell substantially equal to the ground level when the second control signal is low; wherein the drain of the second adjacent cell is coupled to the source of the first adjacent cell.
 9. The biasing and shielding circuit according to claim 8, further comprising: a second connection unit for receiving and outputting the source current of the first adjacent cell to the sense node when the second control signal is high. 